Investigation of the soundness of structures

Abstract

Method and apparatus for the investigation of the soundness of structures of various kinds are disclosed. A method which involves the transmission of a vibration wave through the structure in question and the detection of the frequency spectrum of vibration response at selected locations is disclosed as finding wide application throughout civil and structural engineering investigations. Explanation is given of the adaptation of this method to the continuous monitoring of periodic investigation of structures, with particular reference to offshore oil and gas platforms, to deteriorating concrete structures and components, and to existing bridges, tunnels and railway track systems. A number of elongate gauges for extending longitudinally of selected critical members of an offshore structure below the waterline to monitor its structural integrity and performance are also described.

I claim: 
     
       1. A method of locating and determining the extent of any near surface structural void, shear plane, fault, inclusion zone, weakness or anomaly within a structure said method comprising the steps of: transmitting a vibration wave of momentary fixed frequency over a large frequency range into and through said structure from a specific vibration input location and thereby producing vibration responses at various points in said structure;   detecting the vibration responses at a plurality of detection locations spaced from said vibration input location over at least a substantial two dimensional structure area;   determining the first frequency vibration responses at said detection locations; and   plot connecting the detection location points of maxima or resonance frequency to indicate the presence and to define by contour outline the location, size and nature of a void, shear plane, fault, inclusion, zone of weakness, or anomaly within said structure.   
     
     
       2. A method according to claim 1 for investigating the soundness of a structure, in which said structure consists of soil and rock immediately below the ground surface, wherein said vibration input location and each of said detection locations consist of a respective position on the ground surface, a vibrator being coupled to the ground surface at said vibration input location in a manner to transmit compression waves through the ground; and respective vibration detectors being located on the ground surface at said detection locations to detect reflected vibrations produced by a sub-surface anomaly interrupting passage of the transmitted vibration. 
     
     
       3. A method according to claim 2, for investigating the soundness of soil and rock below the ground surface, wherein a sub-surface anomaly, the presence of which is detected by production of a frequency response spectrum characteristic of such anomaly, has its position and size determined by having the step of detecting the vibration responses at a plurality of detection locations comprising: driving one or more bore holes into the ground in the region of detection of such anomaly;   placing triaxial accelerometers at locations within said one or more bore holes;   triangulating the points of maxima or resonance frequency vibration responses so detected.   
     
     
       4. A method for investigating the soundness of soil and rock below ground surface according to claim 3, in which the frequency of the transmitted vibration is varied within the range of from about 2 Hertz to about 600 Hertz. 
     
     
       5. A method according to claim 1 for investigating the soundness of a structure, in which the structure consists of ground ahead of a forward face of a tunnel being driven below ground surface; and wherein said vibration input location consists of the center of said forward face, a vibrator being coupled thereto to transmit compression waves into said ground ahead of said forward face; and wherein said detection locations consist of positions upon said forward face spaced from said vibration input location, respective detectors being placed thereat to detect the vibration response of said ground ahead of said forward face consisting of said transmitted wave, reflected waves produced by interception of said transmitted wave with anomalies within said ground, and mutual interference between said transmitted and reflected waves. 
     
     
       6. A method according to claim 4 for investigating the soundness of the ground ahead of a forward face of a tunnel being driven below ground surface, in which said tunnel is being driven through rock, and in which the frequency of said transmitted wave is varied in the range from about 10 Hertz to about 2000 Hertz. 
     
     
       7. A method according to claim 5 for investigating the soundness of ground ahead of a forward face of a tunnel being driven below ground surface, in which the tunnel is being driven through soft ground, and in which the frequency of said transmitted wave is varied in the range from about 2 Hertz to about 600 Hertz. 
     
     
       8. A method according to claim 1 for investigating the soundness of a structure, in which the structure consists of sub-surface ground, and wherein: a plurality of bore holes are driven into the ground, said vibration input location and said plurality of detection locations being located within said bore holes; a vibrator unit is located within one said bore hole, the vibrator unit having an expansion chamber which on expansion couples the vibrator unit at said vibration location within the bore hole to the sides of the bore hole, and the vibrator unit having a vibrator adapted to vibrate in a direction parallel to the axis of the bore hole, thereby to generate polarized shear waves radiating through the sub-surface ground from said vibration input location respective vibration response sensors being placed at said detection locations in one or more other bore holes of said plurality to detect the vibration response at said detection locations as the frequency of the transmitted shear wave is varied over said range.   
     
     
       9. A method according to claim 1 for investigating the soundness of a structure, in which the structure consists of sub-strata beneath an existing building, and wherein said vibration input location consists of a position on a basement floor of said existing building, a vibrator being coupled thereto in a manner to produce said vibration wave and to transmit that vibration wave into the sub-strata; and wherein said detection locations consist of locations remote from said vibration input location and selected from the group consisting of locations on the basement floor, locations on walls of the basement and locations on ground surface adjacent said building, respective vibration response detectors being placed at said detection locations and being arraned to detect the vibration response at said detection locations as the frequency of the transmitted wave from said vibrator is varied. 
     
     
       10. for use in investigating the soundness of structues, means for producing and transmitting a vibration through said structure comprising: a vibrator adapted to be set into vibration with a frequency selected from within a range of frequencies;   a ground plate adapted for coupling the vibrator to ground surface, to soil and rock surfaces on a forward face of a tunnel, and the like, the ground plate having a generally saucer-shape with a substantial rim, the concave side of said saucer-shaped ground plate being adapted to bear againt the ground and being divided into a plurality of sections by radial webs;   means for connection of the spaces defined between said radial webs to a source of vacuum for at least partial exhaustion of space defined between the saucer-shaped ground plate and the ground, tunnel forward face, and the like.   
     
     
       11. A vibrator unit for use in investigating the soundness of structures, said vibrator unit being adapted to be lowered to a location within a bore hole within said structure, having an expansion chamber arranged such that upon extension thereof the vibrator unit is coupled at a selected position within the bore hole to the sides thereof, a vibrator within said unit adapted to vibrate in a direction parallel to the axis of the bore hole and to thereby generate polarized shear waves radiating through a sub-surface ground from the region of the vibrator unit, said vibrator unit comprising: a thick-walled cylindrical electrically-insulated shell provided with similar thick-walled end caps tightly and sealingly fitted to the ends of the cylindrical shell;   a pair of end electrodes respectively fixedly located at opposite ends of said shell and extending thereacross;   entirely filling the cylindrical space defined between said electrodes and said cylindrical shell, a gel-like silicon material in which is suspended a plurality of fine magnetically sensitive metallic particles;   means adapted to couple said electrodes to a source of electrical power, passing in fluid tight fashion through one said end cap, and being adapted to provide an electrical current of selected frequency to said electrodes to cause oscillation of said metallic particles between said electrodes to impact an oscillating inertial energy to the gel-like silicon material matrix;   and wherein said expansion chamber comprises an inflatable capsule attached to one side of the exterior of said cylindrical shell, said capsule being provided with means for connection to a pneumatic source for expansion of said capsule within a bore hole to positively locate said vibrator unit at a predetermined location within said bore hole.   
     
     
       12. A method of monitoring the soundness, structural integrity and performance of a structure over a period of time, the method consisting of: producing a vibration wave over a large frequency range;   transmitting said vibration wave through said structure from a vibration location to produce vibration responses therein;   sampling the vibration response at a plurality of detection locations on said structure spaced from said vibration location;   producing a record of the frequencies at which maxima occur in the frequency spectra; and   performing all the aforesaid steps at predetermined intervals of time to determine any variation in frequencies at which maxima occur in the frequency spectra of vibration response at the same plurality of detection locations during the passage of time such that the variation in frequency constitutes the first sign of incipient failure of said structure and wherein the general location of said incipient failure may be determined relative to the detection location by reference to the individual records of said given detection locations.   
     
     
       13. A method according to claim 12 for monitoring the soundness, structural integrity and performance of a structure over a period of time, in which the structure consists of an off-shore structure selected from the group consisting of oil and gas off-shore exploration platforms and oil and gas off-shore production platforms, a vibrator being coupled to said off-shore structure at said vibration location in a manner to transmit compression waves thereinto and therethrough, and respective detectors being coupled to the off-shore structure at said detection locations in a manner to detect the vibration response of said structure at said detection locations; and wherein control means are provided on said structure automatically producing an effective record of the frequencies at which maxima occur in the frequency spectra of vibration response at said detection locations and of any variation in the frequencies of said maxima during the passage of time indicates the first signs of incipient failure of a portion of said off-shore structure. 
     
     
       14. A method according to claim 12 for monitoring the soundness, structural integrity and performance of a structure over a period of time, in which the structure consists of one or more concrete components liable to deteriorate, a vibrator being coupled to said one or more concrete components at said vibration location and respective detectors being located at said detection locations, being positions spaced along the length of said one or more concrete components, to effectively produce a record of the frequencies at which maxima occur in the frequency spectra of vibration response at said detection locations as said vibrator is swept through said range of frequency, and to produce a record of any variation in the frequencies at which maxima occur in the frequency spectra of vibration response at said same detection locations during the passage of time. 
     
     
       15. A method according to claim 12 for monitoring the soundness, structural integrity and performance of a structure over a a period of time, in which the structure consists of an existing bridge, a vibrator being coupled at said vibration location to said bridge in a manner to produce and transmit said vibration wave, and respective vibration response detectors being placed at said detection locations, being locations on said bridge spaced from said vibration location, effectively to produce a record of the frequencies at which maxima occur in the frequency spectra of vibration response at said detection locations and to provide a record of any variation in the frequencies at which maxima occur in the frequency spectra of vibration response at said same detection locations during the passage of time. 
     
     
       16. A method according to claim 12 for monitoring the soundness, structural integrity and performance of a structure over a period of time, in which the structure consists of an existing tunnel a vibrator being coupled at said vibration location to said tunnel in a manner to produce and transmit said vibration wave, and respective vibration response detectors being placed at said detection locations, being locations on said tunnel spaced from said vibration location, effectively to produce a record of the frequencies at which maxima occur in the frequency spectra of vibration response at said detection locations and to provide a record of any variation in the frequencies at which maxima occur in the frequency spectra of vibration response at said same detection locations during the passage of time. 
     
     
       17. A method according to claim 12 for monitoring the soundness, structural integrity and performance of a structure over a period of time, in which the structure consists of a length of railway track system, comprising the track proper, sleepers, and ballast; a vibrator being coupled at said vibration location to a length of said system in a manner to produce and transmit said vibration wave through said length of system; and respective vibration response detectors being placed at and coupled to said system at said detection locations, being locations spaced therealong, to effectively produce a record of the frequencies at which maxima occur in the frequency spectra of vibration response at said detection locations and to provide a record of any vibrations in the frequencies at which said maxima occur at said same detection locations during the passage of time. 
     
     
       18. An off-shore structure provided with apparatus for continued monitoring of soundness, structural integrity and performance thereof over a period of time, which apparatus comprises: at least one vibrator mounted on and coupled to said structure at a vibration location above the water-line and being adapted to produce a vibration with a frequency selected within a frequency range and to transmit said vibration as a wave through said structure from said vibration location;   a plurality of vibration response detectors coupled to said structure at detection locations thereon spaced from said vibration location above the water-line;   means coupled to said plurality of detectors for effectively producing a record of the frequencies at which maxima occur in the frequency spectra of vibration response over said range at said detection locations;   control means for operating said apparatus at predetermined intervals and for producing a record of any variation in the frequencies at which maxima occur in the frequency spectra of vibration response at the same detection locations during the passage of time;   and wherein said apparatus further comprises elongate gauges extending longitudinally of selected critical members of said structure below the water-line, each said gauge having its ends solid with its respective member and including means for causing a vibration to be set up between the ends of the gauge, and means for detecting the characteristic frequency of said vibration.   
     
     
       19. An off-shore structure according to claim 18 wherein said control means is further adapted to periodically operate said gauges and is provided with means for recording any variation of the detected characteristic frequencies of said gauges indicative of changes in the longitudinal dimensions of said members and thus of their strain states. 
     
     
       20. An off-shore structure provided with means for monitoring its structural integrity and performance, which means comprise: elongate gauges extending longitudinally of selected critical members of the said structure below the water-line, each said gauge having its ends solid with its respected member and including means for causing a vibration to be set up between the ends of the gauge, and means for detecting the characteristic frequency of said vibration;   means for periodically operating said gauges from a location above the water-line; and   means for recording any variation of the detected characteristic frequencies of said gauges indicative of changes in the longitudinal dimensions of said members and thus of their strain states.   
     
     
       21. In an off-shore structure according to claim 20 provided with means for monitoring its structural integrity and performance, an elongate gauge comprising: a wire stretched between opposite ends of means defining a tubular cavity extending along at least the greater part of one said critical member, with at least the ends of said means solid with the respective member;   a plurality of equi-spaced electromagnetic means along the length of the wire and adapted when operated in unison to determine positions for antinodes in vibration of the wire in an harmonic mode; and   an electrical control and detection circuit coupled to all said electromagnetic means and adapted to excite said electromagnetic means simultaneously to set said wire into its said harmonic and further adapted to record the resultant characteristic frequency of the said harmonic.   
     
     
       22. In an off-shore structure according to claim 20 provided with means for monitoring its structural integrity and performance, an elongate gauge which comprises: a stretched wire located within means defining a tubular cavity extending along at least the greater part of one said critical member with at least the ends of said means solid with the respective member;   an impact device adapted to impact means supporting said wire at one end thereof, thereby to set said wire into a longitudinal mode of vibration; and   means for detecting the double transit time of a longitudinal wave propagated along the length of said wire.   
     
     
       23. In an off-shore structure according to claim 20 provided with means for monitoring its structural integrity and performance, an elongate gauge which comprises: an elongate tube extending along at least the greater part of the length of one said critical member;   at one end of said tube and coupled thereto a square-wave vibrator adapted to provide a shock wave to propagate along the said tube;   the other end of the tube being open to the sea; and   detector means coupled to said tube and effective to record the characteristic water-hammer frequency of said gauge.   
     
     
       24. In an off-shore structure according to claim 20 provided with means for monitoring its structural integrity and performance, an elongate gauge which comprises: an elongate tube extending along at least the greater part of the length of one said critical member;   at one end of said tube and coupled thereto a square-wave vibrator adapted to provide shock wave to propagate along the said tube;   the other end of the tube being closed; and   detector means coupled to said tube and effective to record the characteristic water-hammer frequency of said gauge.